Conductive transparent probe and probe control apparatus

ABSTRACT

A conductive transparent probe used in a probe control apparatus for adjusting a distance between the apex of the probe and a sample by vibrating the probe with an vibrator in a direction perpendicular to the axis of the probe is provided. The conductive transparent probe includes: an optical fiber having a taper part at one end; a conductive transparent film formed on the surface of the taper part; a first metal film formed on the surface of the optical fiber other than the taper part; wherein the conductive transparent film and the first metal film are electrically connected, and length and thickness of the first metal film are determined such that the conductive transparent probe vibrates while contacting with the vibrator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. application Ser.No. 10/228,139 filed Aug. 27, 2002 now U.S. Pat. No. 6,953,930, andbased upon and claims the benefit of priority to Japanese PatentApplication Nos. 2001-256206, filed Aug. 27, 2001; and 2001-341651,filed Nov. 7, 2001, the entire contents each of which are incorporatedherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a conductive transparent probe and aprobe control apparatus. More particularly, the present inventionrelates to a conductive transparent probe used in a tunnelingluminescence microscope, and a probe control apparatus for controlling adistance between the apex of a probe and a sample, wherein the tunnelingluminescence microscope measures optical and electronic characteristicsof a very small region of a size of the nanometer order by detectingluminescence caused by applying a probe current into the sample.

2. Description of the Related Art

As devices become small and technologies for utilizing characteristicsof individual molecules develop, great demands have arisen fortechnologies for characteristic evaluation of a very small region of asize of the nanometer order in materials (to be referred to as a nanoregion hereinafter), and for technologies for optical and electroniccharacteristic measurement of individual molecules intrinsically havinga size of the nanometer order. For realizing such measurement andevaluation, a tunneling luminescence microscope (to be referred to as aTL microscope hereinafter) is provided that enables detection andanalysis of luminescence caused by applying a current from an apex of asharpened probe to a sample. In addition, a probe that is transparentand has conductivity (to be referred to as a conductive transparentprobe hereinafter) has been developed, wherein the conductivetransparent probe applies a current from its apex into a sample, and atthe same time, receives and collects luminescence from the apex, so thatluminescence collection yield is improved. The conductive transparentprobe is powerfully used for characteristic evaluation of a nano region.As effectiveness of the TL apparatus for characteristic evaluation of anano region increases, it is demanded by users that the sample to bemeasured is not only a material having only a conductive region but alsoa material in which a nonconductive region or a highly resistive regionis mixed with the conductive region.

In an apparatus (to be referred to as a probe microscope hereinafter)that measures a sample by bringing a probe extremely close to thesurface of the sample, it is very important to properly control a verysmall distance (to be referred to as a gap hereinafter) between the apexof the probe and the surface of the sample. Therefore, generally, as forthe probe microscope (for example, a scanning tunneling microscope (tobe referred to as an STM, hereinafter)) that utilizes a tunnelingcurrent flowing between the probe and the sample for measurement, amethod of detecting the tunneling current flowing between the probe andthe sample is used for controlling the gap (this control method iscalled an STM control method hereinafter). The reason for using thismethod for realizing precision gap control is that the tunneling currentis very sensitive to the gap.

However, the STM control method can be applied only to a sample of whichthe whole region is electronically conductive, and the STM controlmethod cannot be applied to a sample in which a nonconductive region ora highly resistive region is mixed. Therefore, a TL apparatus thatenables gap control without using the tunneling current is desperatelydesired, such that the TL apparatus can be applied to a sample in whicha nonconductive region or a highly resistive region is mixed.

As a gap control method without using the tunneling current, there is amethod for utilizing an atomic force such as attractive force andrepulsive force between the apex of the probe and the sample. In thismethod, when the apex of the probe approaches very close to the surfaceof the sample, atomic force between the apex and the surface isdetected, and the gap is adjusted such that the detected value becomesconstant.

For feeding back the detected value for performing gap control, there isa method of using an optical lever and a soft probe of a cantilevershape.

In this case, a laser beam is used for detecting a very smalldisplacement of the probe. However, since the laser beam is extremelystronger than a detected signal light used for observing the sample,there is a problem in that the SN ratio decreases when measuring weakluminescence caused by the tunneling current.

It is desirable to use a leaner probe made of an optical fiber in orderto suppress optical transmission loss in the probe. However, it isdifficult to use such a probe as the soft probe of a cantilever shapethat is necessary for realizing an optical lever.

In addition, there is a method called a shear force gap control method.In the method, a linear probe perpendicular to the surface of the sampleis vibrated in a direction perpendicular to a center axis of the probe,so that atomic force is detected by measuring amplitude of the probevibrating at a specific frequency. In this method, when a voltage isapplied between the apex of the probe and the sample for causingluminescence, a current flows into a sensor used for detecting theamplitude, so that a detected signal is disturbed and gap controlbecomes unstable. Therefore, there is a problem in that a voltage cannotbe applied between the probe and the sample when the shear force gapcontrol method is used.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a conductivetransparent probe that is applicable to the shear force gap controlmethod while the tunneling current can be applied to a very small regionwithout decreasing luminescence collection yield. In addition, anotherobject of the present invention is to provide a probe control apparatusfor applying a voltage between the apex of the probe and a sample so asto apply a current from the apex of the probe to cause luminescence fromthe sample, wherein the probe control apparatus is applicable to asample in which a nonconductive region or a highly resistive region ismixed with a conductive region, a conductive transparent probe can beused as a probe, and it is not necessary to use a laser beam thatdecreases the SN ratio when weak luminescence caused by tunnelingcurrent is measured.

The above-mentioned object is achieved by a conductive transparent probeused in a probe control apparatus for adjusting a distance between theapex of the conductive transparent probe and a sample by vibrating theconductive transparent probe with a vibrator in a directionperpendicular to the axis of the conductive transparent probe, theconductive transparent probe includes:

an optical fiber having a taper part at one end;

a conductive transparent film formed on the surface of the taper part;

a first metal film formed on the surface of the optical fiber other thanthe taper part;

wherein the conductive transparent film and the first metal film areelectrically connected, and length and thickness of the first metal filmare determined such that the conductive transparent probe vibrates whilecontacting with the vibrator.

According to the above-mentioned conductive transparent probe accordingto the present invention, shear force gap control can be performedwithout losing functions of applying a probe current and collectingluminescence, and measurement by using luminescence can be performedstably even for a sample in which a nonconductive region or a highlyresistive region is mixed with a conductive region.

The above object is also achieved by a probe control apparatusincluding:

a probe that is straight and vertical with respect to a surface of asample;

a vibrator for vibrating the probe in a direction perpendicular to acenter axis of the probe;

an amplitude detection part for detecting an amplitude of the probe;

a part for controlling a distance between the apex of the probe and thesample by controlling the amplitude of the probe vibrating at a specificfrequency to be a predetermined amplitude;

a voltage applying part for applying a voltage between the apex of theprobe and the sample;

wherein the probe has optical transparency and electrical conductivity,and the probe is electrically insulated from the amplitude detectionpart.

According to the above-mentioned probe control apparatus according tothe present invention, gap control between the probe and the sample canbe performed stably even for a sample in which a nonconductive region ora highly resistive region is mixed with a conductive region, for whichsample it is difficult to perform gap control by using probe current.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 shows a block diagram of a probe control apparatus of the presentinvention;

FIG. 2A shows a relative position of the apex of a probe 1 and a sample3 of FIG. 1;

FIG. 2B shows a status in which the apex of the probe 1 movessinusoidally with respect to the time axis;

FIG. 3 shows a horizontal section of the first embodiment of aconductive transparent probe of the present invention;

FIG. 4 shows a basic structure of a shear force gap control system inwhich the conductive transparent probe of the present invention isimplemented;

FIG. 5 shows a horizontal section of the second embodiment of aconductive transparent probe of the present invention;

FIG. 6 shows a horizontal section of the third embodiment of aconductive transparent probe of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be descriedwith reference to figures.

In the embodiments, a probe control apparatus will be described first,and details of a conductive transparent probe applicable to the probecontrol apparatus will be described next.

(Probe Control Apparatus)

FIG. 1 shows a block diagram of the probe control apparatus of thepresent invention. As shown in the figure, the probe control apparatusof the present invention includes a probe 1, a vibrator 7, a powersource for vibration 9, a sensor 10, a sensing signal processing circuit11, a sample position driving mechanism 12, a control circuit 13 for thesample position driving mechanism, a probe bias power source 14, aconductive holding plate 15 and a supporting structure 16.

The apex of the linear probe 1 mounted perpendicular to the surface ofthe sample 3 is tapered to a point. The probe 1 is made of a linearoptical fiber having optical transparency. A conductive film is appliedon the periphery and the taper part of the optical fiber to provideconductivity, wherein the conductive film applied on the taper part isoptically transparent for providing a luminescence collection ability.The probe 1 is held by the conductive holding plate 15 apart from theapex of the probe by 1–2 cm, so that the probe 1 is mounted on thesupporting structure 16.

The conductive holding plate 15 is connected to one end of the probebias power supply 14, and supplies a current from the probe bias powersupply 14 to the probe 1. The other end of the probe bias power supply14 is connected to the sample 3, so that a voltage applying mechanismfor applying voltage between the apex of the probe 1 and the sample 3 isformed.

The vibrator 7 for vibrating the probe 1 in a direction perpendicular tothe center axis of the probe 1 is provided on the supporting structure16. The vibrator 7 is connected to the power supply for vibration 9, andpushes a point apart from the apex of the probe 1 by several millimetersvia a sensor 10 that is an amplitude detection means, so that the probe1 is vibrated in the direction parallel to the surface of the sample 3.

The sensor 10 detects the amplitude of vibration of the probe 1, andoutputs a voltage value in proportion to a displacement amount(amplitude). The output from the sensor is transmitted to the sampleposition driving mechanism 12 via the sensing signal processing circuit11 and the control circuit 13.

The sample position driving mechanism 12 receives an output from thecontrol circuit 13, and moves the sample.

The sensing signal processing circuit 11, the control circuit 13 and thesample position deriving mechanism 12 form a distance control means forcontrolling a distance (gap 4) between the apex of the probe 1 and thesample 3.

An insulator 81 electrically insulates the probe 1 from the sensor 10,and an insulator 82 electrically insulates the sensor 10 from thevibrator 7.

An operation of the probe control apparatus of the present invention isas follows.

The probe 1 is placed on the sensor 10, and is vibrated by the sensor 10in the direction perpendicular to the center axis of the probe 1 at aresonance frequency. The sensor 10 outputs a voltage corresponding tovibration of the probe 1. At frequencies near the resonance frequency,if the frequency changes slightly, the amplitude of the probe 1 changesgreatly. Thus, the amplitude of the probe 1 is monitored with thevibrator 10 for sensing at a frequency slightly apart from the resonancefrequency. In this status, the apex of the probe 1 approaches thesurface of the sample 3 (the operation for the probe 1 approaching thesample 3 is referred to as “approach”).

Even after starting the approach, while the gap 4 is so large thatatomic force between the probe 1 and the sample 3 can be neglected, theprobe 1 continues to vibrate at a constant frequency and a constantamplitude. Therefore, the amplitude of voltage output from the sensor 10is constant, since the voltage change corresponds the vibration.

Next, when the probe 1 further approaches the surface of the sample 3 soclosely that atomic force becomes large, the atomic force acts as aresistance (a shear force) to the vibration of the probe 1, and thefrequency changes. Therefore, the amplitude monitored by the sensor 10changes. When the amplitude of the probe 1 changes, output voltage dataof the sensor 10 also change. When the amplitude of the output voltagebecomes a predetermined value, approach of the probe is stopped. Afterthat, the gap 4 between the probe 1 and the surface of the sample isadjusted by performing feedback control such that the amplitude of theprobe 1 is constant.

If a current flows to the sensor 10 from the probe 1 when applying avoltage between the apex of the probe 1 and the sample 3, the detectedsignal output from the sensor 10 is disturbed and gap control becomesunstable. For preventing this signal disturbance, the insulator 81 isinserted between the probe 1 and the sensor 10, so that they areelectrically insulated.

The atomic force occurs irrespective of whether the sample 3 isconductive or nonconductive. Therefore, gap control between the probe 1and the surface of the sample 3 can be performed even when the sample 3includes both a conductive region and a nonconductive region or a highlyresistive region. For example, when the probe 1 is placed above thenonconductive region of the sample 3, the gap 4 between the probe 1 andthe surface of the sample 3 is controlled properly by using atomic forcealthough the probe current does not flow. When the probe 1 is placedabove the conductive region of the sample 3, gap control is performed byusing the atomic force, and, in addition, tunneling current andluminescence caused by the current can be detected since the probecurrent can be applied.

That is, tunneling current and luminescence caused by the tunnelingcurrent can be measured even for a sample in which a nonconductiveregion or a highly resistive region is mixed with a conductive region,for which sample it is difficult to control the gap 4 by using thetunneling current. In addition, since detection of the gap 4 isperformed by the sensor 10 so that a laser beam is not used, the SNratio of the detected signal light is not lowered when measuring weakluminescence caused by the tunneling current. Therefore, measurementwith a high SN ratio can be achieved.

In addition, since it is not necessary to use a soft probe of acantilever shape, a probe made of a linear optical fiber applicable toforming a conductive transparent probe can be used.

Further, since the insulator 81 is inserted between the probe 1 and thesensor 10 so as to electrically insulate the sensor 10 from the probe 1,current does not flow to the sensor 10 from the probe 1 even when avoltage is applied between the probe 1 and the sample 3. Thus, thedetected signal is not disturbed, so that gap control is performedstably.

FIGS. 2A and 2B show a relationship between an amplitude B–B′ ofvibration of the apex of the probe 1 in the horizontal direction and ameasurement target region A–A′ of the sample 3. FIG. 2A shows a relativeposition of the apex of the probe 1 and the sample 3. FIG. 2B shows astatus in which the apex of the probe 1 moves sinusoidally with respectto the time axis.

In the probe control apparatus of the present invention, since the gap 4is controlled by using the atomic force between the probe 1 and thesample 3, it is necessary that the shear force caused by the atomicforce acts on the probe 1 sufficiently. Therefore, it is difficult tolessen the horizontal amplitude B–B′ of the apex of the probe 1 to avalue less than several tens of nanometers. Therefore, spatialresolution in measurement is limited by the amplitude B–B′.

In this embodiment, to avoid such limitation, the current applied to thesample 3 from the probe 1 is applied like a pulse in synchronizationwith the phase of vibration of the apex of the probe 1. The timing forapplying current can be synchronized with any phase. For example, inthis embodiment, a pulse voltage is applied from the probe bias powersource 14 while the apex of the probe 1 is located in the measurementtarget region A–A′ which is near the center O of the amplitude.Accordingly, even when the amplitude B–B′ of the apex of the probe 1 inthe horizontal direction necessary for controlling the gap 4 is large,the spatial resolution of measurement by tunneling current andluminescence of the tunneling current can be intensified according tosmallness of the measurement target region A–A′.

As mentioned above, according to the probe control apparatus of thepresent invention, the probe control apparatus is applicable to a samplein which a nonconductive region or a highly resistive region is mixedwith a conductive region, and a conductive transparent probe can be usedwithout using a laser beam, which lowers the SN ratio when measuringweak luminescence caused by tunneling current. In addition, a voltagecan be applied between the apex of the probe and the sample for applyinga current from the apex of the probe to the sample to causeluminescence.

Since the voltage applying mechanism applies a pulse voltage between theapex of the probe and the sample in synchronization with vibration ofthe probe, spatial resonance for measurement by using tunneling currentand luminescence of the tunneling current can be intensified even if theamplitude of the apex of the probe in the horizontal direction necessaryfor controlling the gap is large.

(Conductive Transparent Probe)

Next, a conductive transparent probe applicable for use in theabove-mentioned probe control apparatus will be described.

In order to perform gap control stably by the shear force gap control byusing the above-mentioned probe control apparatus, it is necessary forthe probe to have a smooth frequency-to-amplitude characteristic(represented by a curve indicating a relationship between frequency andamplitude) with few parasitic vibrations. For realizing thischaracteristic, it is necessary that the probe and the vibrator beintegrated while vibrating, so that the probe vibrates by followingfaithfully the vibration of the vibrator. In order that the probe andthe vibrator vibrate together, it is necessary that the probe has amoderate rigidity for keeping moderate contacting pressure between theprobe and the vibrator, and that the probe has a moderate elasticity tovibrate stably. If the probe is so soft that contacting pressure betweenthe probe and the vibrator is small, the probe vibrated by the vibratorjumps (amplitude of the probe exceeds that of the vibrator) from thevibrator, so that the probe does not vibrate together with the vibratorand does not follow faithfully the vibration of the vibrator, andparasitic vibration occurs. Thus, movement of the probe becomesunstable. If the probe is so rigid that the contact pressure is toolarge, the probe may be broken, or the probe cannot be vibrated at thedesired amplitude, so that proper movement cannot be obtained.

However, a conventional probe used for STM is short, and a thick metalplating is applied on the surface of the probe for preventing mechanicalvibration that may cause noise. Thus, rigidity of the probe is large, sothat rigidity and elasticity are not proper for realizing shear forcegap control. Therefore, the probe cannot be used for shear force gapcontrol. Therefore, a conductive transparent probe is used as follows inthe present invention.

FIG. 3 shows a horizontal section of the first embodiment of theconductive transparent probe of the present invention.

The conductive transparent probe is made of an optical fiber 21including a core 22 and a cladding 23. A taper part 25 is provided inthe optical fiber 21, wherein the taper part 25 ranges within severalhundred micrometers from one end opposed to a sample 32 in the opticalfiber 21, and the apex of the taper part 25 is sharpened to a size ofthe nanometer order. To provide conductivity and a luminescencecollection function to the taper part 25 of the nonconductive opticalfiber 21, a conductive transparent film 24 having conductivity andtransparency is applied on the surface of the taper part 25. Inaddition, in order to provide conductivity to the optical fiber 21, afirst metal film 26 having conductivity is applied on the outer surfaceof the optical fiber 21. The conductive transparent film 24 and thefirst metal film 26 are connected electrically.

Tunneling current is applied from the apex of the conductive transparentprobe to the sample 32, and luminescence caused by the tunneling currentis collected from the apex of the same conductive transparent probe.

A part ranging from a point apart from the one end by a distance D tothe other end of the optical fiber 21 is held by a conductive holdingplate 29 formed by a metal plate, for example, so that the conductivetransparent probe is mounted on the supporting structure 31 (refer toFIG. 4). The conductive holding plate 29 is connected to a bias powersource (not shown in the figure). Since the holding plate 29 contactsthe first metal film 26 of the conductive transparent probeelectrically, it has a function to provide a current to the conductivetransparent probe.

The current from the bias power source is supplied to the apex of theconductive transparent probe via the first metal film 26 on the surfaceof the optical fiber 21 and the conductive transparent film 24 on thetaper part 25.

The conductive transparent probe is vibrated in a directionperpendicular to the axis of the optical fiber 21 by using a sensor 28that pushes a point (vibration point) apart from the one end (apex) by adistance d (d<D, about several millimeters). The sensor 28 is attachedto a vibrator 30 (FIG. 4), and has a function to transmit vibration ofthe vibrator 30 to the conductive transparent probe and a function todetect vibration and amplitude of the conductive transparent probe. Thesensor 28 is electrically insulated from the conductive transparentprobe.

When the vibrator 30 operates, the conductive transparent probe vibratesat a frequency and an amplitude corresponding to those of a cantileverof a length D. In order to transmit vibration of the vibrator 30 to theapex of the conductive transparent probe faithfully, it is desirable toshorten the distance d. In addition, in order for the conductivetransparent probe to vibrate easily, it is desirable to set thevibration point apart from the part where the conductive transparentprobe is held, and to make the distance D as large as possible.According to an experiment, stable operation was obtained and the probewas easy to handle when the distance D was no less than 5 mm and thedistance d was about 2–3 mm, wherein the distance D is almost the sameas the length of the first metal film 26 from a part adjacent to thetaper part 25 to the other end.

Next, operation of a shear force gap control system in which theconductive transparent probe of the present invention is implementedwill be described.

FIG. 4 shows a basic structure of the shear force gap control system inwhich the conductive transparent probe of the present invention isimplemented. Although the shear force gap control system is similar tothe probe control apparatus described by using FIG. 1, the structure issimplified in the following embodiments since the conductive transparentprobe is mainly described.

As shown in the figure, a part of the other end side of the conductivetransparent probe is held by the conductive holding plate 29. At thistime, the conductive transparent probe is placed on the back side of thesensor 28. Next, the conductive transparent probe is bent a little, andthe conductive transparent probe is put on the sensor 28 such that apoint a distance d apart from the apex of the conductive transparentprobe is placed on the front of the sensor 28. By elastic force causedby the bending, the conductive transparent probe and the sensor 28contact each other with moderate contacting pressure.

Next, the conductive transparent probe is vibrated in a directionperpendicular to the axis of the optical fiber 21 (that is, parallel tothe surface of the sample) at a specific frequency. The vibratedconductive transparent probe vibrates as a cantilever having a fixed endthat is the part attached to the conductive holding plate 29. The sensor28 outputs a voltage corresponding to the amplitude of vibration of theconductive transparent probe.

While the conductive transparent probe is vibrated at a frequencyslightly different from an Eigen frequency, when the frequency ischanged slightly, the amplitude of the conductive transparent probechanges greatly. Thus, the conductive transparent probe is vibrated at afrequency slightly different from the Eigen frequency, and the amplitudeis monitored by the sensor 28.

When atomic force between the conductive transparent probe and thesample 32 becomes large as the conductive transparent probe approachesthe sample 32, the atomic force acts on the conductive transparent probeas a shear force in a direction perpendicular to the axis of the opticalfiber 21. The shear force acts as a resistance force against vibrationof the conductive transparent probe vibrating as a cantilever. Thus, thefrequency of the conductive transparent probe changes slightly so thatthe amplitude of the conductive transparent probe at a monitoredfrequency is changed. This change of the amplitude is detected as achange of output voltage of the sensor 28. When the amplitude of theoutput voltage becomes a predetermined value, that is, when the shearforce becomes a predetermined value, the approach of the conductivetransparent probe to the sample 32 is stopped. After that, the gapbetween the conductive transparent probe and the sample 32 is controlledsuch that the amplitude of the conductive transparent probe at themonitored frequency is constant (that is, such that shear force isconstant) while performing measurement. Accordingly, stable operation ofan AFM (Atomic Force Microscope) can be obtained, wherein the AFM is amicroscope performing the gap control by using atomic force (Yang et al.“Near-field differential scanning optical microscope with atomic forceregulation”, Appl. Phys. Lett., 60(24), 15 Jun. 1992, for example, canbe referred to for more information on conventional AFM).

Since the shear force gap control is stably performed irrespective ofconductivity of the sample 32, it becomes possible to realize a TLapparatus using the conductive transparent probe, that is applicable toa sample in which a nonconductive region or a highly resistive region ismixed with a conductive region.

For satisfying a contacting condition between the conductive transparentprobe and the sensor 28 necessary for conducting stable shear force gapcontrol, thickness and length of the first metal film 26 applied on theouter surface of the optical fiber 21 are adjusted, so that flexuralrigidity of the conductive transparent probe is adjusted. As a materialof the first metal film 26, nickel, stainless steel and the like can beused, for example. However, any other material can be used as long asadhesive force between the material and the surface of the optical fiber21 is strong and conductivity is high.

If the distance D from the end of the conductive transparent probe tothe conductive holding plate 29 is equal to or less than severalmillimeters, the flexural rigidity of the optical fiber 21 becomeslarge. Therefore, there occurs a case in which the sensor 28 slides onthe surface of the conductive transparent probe, so that vibrations ofthe vibrator 30 do not transfers to the optical fiber 21 faithfully.

When the thickness of the first metal film 26 of the outer surface ofthe optical fiber 21 is smaller than about 0.2 μm, the optical fiber 21is easily broken by a slight shear force. In addition, if first metalfilm 26 is thin, electrical resistance from the holding plate 29 to theapex of the optical fiber 21 becomes large, so that it becomes difficultto supply a current to the apex of the optical fiber 21. Therefore, aconductive transparent probe having a thin first metal film 26 is notpractical.

On the other hand, if the thickness of the first metal film 26 isgreater than 10 μm, rigidity of the conductive transparent probe becomeslarge, so that a large force is necessary for bending the conductivetransparent probe. When bending the conductive transparent probeforcibly, plastic deformation occurs so that the shape does not returnto its original shape. Therefore, the conductive transparent probehaving a thick first metal film 26 is not applicable to the shear forcegap control.

According to an experiment, when thickness of the first metal film 26was 0.2–10 μm, the conductive transparent probe 21 had elasticity properfor shear force gap control, and good electrical conductivity, so thatthe conductive transparent probe had good characteristics for shearforce gap control.

As mentioned above, when the length of the first metal film 26 from thepart adjacent to the taper part 25 to the other end is equal to orgreater than 5 mm, and thickness of the first metal film 26 is 0.2–10μm, contacting pressure between the conductive transparent probe and thesensor 28 becomes a proper value, so that the conductive transparentprobe vibrated by the sensor 28 does not jump from the sensor 28. Theconductive transparent probe integrates with the sensor 28, and followsvibration of the sensor 28 faithfully. The movement of the conductivetransparent probe does not become unstable due to parasitic vibrationand the like. The conductive transparent probe is not too stiff andcontact pressure is not too large. In addition, the conductivetransparent probe does not break, and is oscillated at the desiredamplitude. Thus, the conductive transparent probe operates properly.

Stress concentrates on a boundary part between a part performing bendingvibration as a cantilever and a part held by the conductive holdingplate 29. Thus, the boundary part is easily broken. In addition, theoptical fiber 21 may be distorted by pressure applied to the conductivetransparent probe from the holding plate 29 for fixing the conductivetransparent probe, so that there is a case that optical characteristicsof the conductive transparent probe degrade.

In a second embodiment of the present invention shown in FIG. 5, asecond metal film 33 is formed on the side of the other end of theconductive transparent probe, such that the optical fiber 21 is notdistorted by a pressure applied to the optical fiber 21 from the holdingplate 29, wherein the thickness of the second metal film 33 is largerthan that of the first metal film 26. As a result of an experiment, itwas found that the thickness of the second metal film 33 needed to be noless than 10 μm, and preferably no less than 50 μm. In this embodiment,if the thickness of metal film between the first metal film 26 and thesecond metal film 33 changes discontinuously, there is a possibilitythat the conductive transparent probe will be broken since stressconcentrates on the part where the thickness changes discontinuously.Therefore, a transitional part 34 where thickness of metal film changessmoothly is provided between the first metal film 26 and the secondmetal film 33. By adopting such a structure, the conductive transparentprobe can be mounted firmly with reliability by the holding plate 29without degrading optical characteristics of the conductive transparentprobe. In addition, a conductive transparent probe that is hard to breakby stress concentration can be realized.

FIG. 6 is a section view of the third embodiment of the conductivetransparent probe of the present invention. As shown in the figure, inthis embodiment, the taper part 25 provided in one end part of theconductive transparent probe is covered with a material 35 through whichlight cannot pass, and a very small hole is provided at the apex of thetaper part 25 opposed to the sample 32. By adopting such a structure, itbecomes possible to selectively collect only near optical fields in thetunneling current luminescence.

In the above-mentioned configuration of the probe control apparatus forcontrolling the gap by using shear force, vibration of the vibrator isapplied to the conductive transparent probe via the sensor contactingthe conductive transparent probe, and the sensor detects changes, due toatomic force, of amplitude of the conductive transparent probe. However,the configuration is not limited to this example. There is followinganother configuration of the gap control apparatus for using shearforce. That is, instead of fixing the conductive transparent probe tothe holding plate 29, the conductive transparent probe can be fixeddirectly to the vibrator, and a laser beam is directed to the conductivetransparent probe from the side direction of the conductive transparentprobe, and, change of amplitude of the conductive transparent probe isdetected by measuring the laser beam modulated by vibration of theconductive transparent probe. The conductive transparent probe can beapplied to an apparatus for performing shear force gap control by suchmethod using a laser beam.

As mentioned above, according to the conductive transparent probe,length and thickness of the first metal film from a part adjacent to thetaper part to the other end are set to values such that the conductivetransparent probe vibrates while integrating with the vibrator.Therefore, it becomes possible to realize a conductive transparent probeapplicable to shear force gap control while tunneling current can beapplied and luminescence collection yield is not degraded. Thus, thepresent invention produces the effect of enabling stable TL measurementfor a sample in which a nonconductive region or a highly resistiveregion is mixed with a conductive region.

Especially, a conductive transparent probe having bending rigidityapplicable to shear force gap control can be realized by setting thelength of the first metal film to be no less than 5 mm, and setting thethickness of the first metal film to be 0.2–10 μm.

In addition, in the conductive transparent probe of the presentinvention, a second metal film is formed on an outer surface of theother end side, wherein thickness of the second metal film is no lessthan 10 μm, and the first metal film and the second metal film areconnected by using a transitional part whose thickness changescontinuously. Therefore, the conductive transparent probe can be heldfirmly with high reliability without degrading optical characteristics,and the conductive transparent probe is not broken even when bendingstress concentrates on a part.

Further, in the conductive transparent probe, the taper part is coveredby a material through which light cannot pass, and a very small hole isprovided on the apex of the taper part covered by the material, whereinthe diameter of the hole is smaller than a wavelength of a transmissionlight. Accordingly, only near optical fields can be collected.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the invention.

1. A probe, comprising: an optical fiber having a tapered end forming anapex; an electronically conductive transparent film formed on a surfaceof the tapered end; and a first metal film formed on a first surface ofsaid optical fiber other than the surface of the tapered end andelectrically connected to said electrically conductive transparent film,wherein said probe is configured for use with a probe control apparatusthat controls a distance between said probe and a sample via a shearforce gap control method.
 2. The probe as claimed in claim 1, whereinthe length of said first metal film is no less than 5 mm and thethickness of said first metal film is from 0.2 μm to 10 μm.
 3. The probeas claimed in claim 2, further comprising: a second metal film formed ona second surface of said optical fiber other than the surface of thetapered end, said second metal film being no less than 10 μm thick; anda transitional metal film formed on a third surface of said opticalfiber other than the surface of the tapered end and connecting saidfirst metal film with said second metal film, said transitional metalfilm having a thickness that continuously increases along a directionfrom said first metal film to said second metal film.
 4. The probe asclaimed in claim 1, further comprising: a material configured to preventtransmission of light from said electrically conductive transparent filmto portions of the tapered end other than the apex.